Optical System for Direct Imaging of Light Markable Material

ABSTRACT

An imaging system. An array of light sources and an array of lenses corresponding to the light sources and having optical axes substantially parallel to one another are provided. The lenses produce collimated output beams. An afocal optical relay having an optical axis substantially parallel to the optical axes of the lenses is also included, the array of lenses being positioned relative to the afocal optical relay so as to form an optical system that produces an image of each collimated output beam on an image plane, each image having a prescribed depth of focus and spot size. The light sources preferably are lasers producing an array of respective laser beams having high intensity and a long waist. A system for writing information on a light-sensitive label includes the imaging system. Methods of imaging and of writing information on a light-sensitive label are also provided.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/477,874 filed Jun. 3, 2009 which is a continuation-in-part of U.S.patent application Ser. No. 11/511,103, filed Aug. 28, 2006, andpublished as U.S. Patent Publication No. 2007/0068630 on Mar. 29, 2007,which claimed priority to Provisional Patent Application No. 60/789,505,filed Apr. 4, 2006, and to Provisional Patent Application No.60/712,640, filed Aug. 29, 2005, and was a continuation-in-part of U.S.patent application Ser. No. 11/069,330, filed on Mar. 1, 2005, now U.S.Pat. No. 7,168,472, which claimed priority to Provisional PatentApplication No. 60/549,778, filed Mar. 3, 2004, all of which areincorporated herein by reference.

TECHNICAL FIELD

The embodiments of the present invention disclosed herein relategenerally to the field of precision laser direct imaging of lightmarkable media used in a printing application, and particularly towriting produce labels “on the fly,” with variable, item-specificinformation, as the labels are about to be applied thereto.

BACKGROUND

Automatic labeling is of interest to the produce industry, in which ithas become a common practice to label each item of produce with someitem-specific information, printed in the form of, for example, text ora bar code. The information about the produce may include, for example,its type, size, date harvested, geographic origin, and whether or notthe produce is organic. In particular, it has become desirable to labeleach item with a Price Look. Up (“PLU”) number, which enables retailersto facilitate quick handling and accurate pricing of produce atcheckout. However, in the past, labeling items with different PLUnumbers, for example, denoting “small,” “medium,” or “large” sizedesignations for apples, has required three separate labeling machines,three separate label designs, and three label inventories. Consequently,it has become desirable to be able to apply variable, programmable,information “on the fly” to a produce label tailored to an individualitem, thereby requiring only a single labeling machine and only asingle, at least partially blank, label design. More backgroundregarding this approach can be found at col. 1 line 11 through col. 2line 45 of Hirst et al., U.S. Pat. No. 7,168,472, entitled Method andApparatus for Applying Variable Coded Labels to Items of Produce, whichissued Jan. 30, 2007 (hereinafter “Hirst”), the entire disclosure ofwhich is hereby incorporated by reference herein, and at paragraphs 2-21of Griffen et al., U.S. Patent Application Publication No. 2007/0068630,entitled Multi-Layer Markable Media and Method and Apparatus for UsingSame, which was published Mar. 29, 2007 (hereinafter “Griffen”), theentire disclosure of which is also hereby incorporated by referenceherein.

As disclosed in both Hirst and Griffen, it is desirable to writevariable information directly onto a label using a light beam. To dothis in a rapid, consistent, and cost effective manner presentschallenges arising from the relationships between the labeling machine,label material, and light beam optics. In particular, it is desirable toprovide a high power light beam so as to reduce the required labelexposure time. It is also desirable to provide a light beam that has along depth of focus at the label so as to ensure that a focused imagewill be written on the label despite potentially significant variationsin the label position, relative to the nominal image surface of thelight beam optics. It is further desirable to minimize aberrations inthe light beam to provide, as nearly as practical, a diffraction limitedlight beam image at that image surface.

One method and apparatus for direct writing of a pattern with a laserbeam is described in Tamkin, U.S. Pat. No. 6,084,706 (hereinafter“Tamkin”). Tamkin discloses a three-mirror afocal optical system inwhich the mirrors may have aspheric (e.g., parabolic, hyperbolic, orelliptical) or spherical surfaces. Such an all-reflective architecture,which uses mirrors instead of lenses throughout, achieves a high levelof transmission efficiency compared to a lens-based system, in which thelens medium inevitably absorbs significant light energy at certainwavelengths.

In general, an afocal optical system is an optical system in which boththe object and the image are assumed to be located at infinity. Lightrays entering and leaving an afocal optical system are parallel.Examples include binoculars and telescopes, in which the image, althoughmagnified by the optical system, is focused by the eye. Magnificationmay increase or decrease (i.e., fractionally magnify) the size of theimage, depending on whether a magnification factor is greater than orless than one, respectively. An afocal optical system may be formed bycombining two focal optical systems so that the rear focal point of thefirst system coincides with the front focal point of the second system,yielding an overall system that has no effective focal length. Severalembodiments of a three-mirror afocal system are described in Tamkin,each having different magnifications.

In Tamkin, a single laser source and a beam splitter are used to produceup to eight separate beams, which are then passed through an opticalsystem to produce a 15,000-pixel image, having pixel sizes in the rangeof about 1-10 microns. The three-mirror afocal system is then used torelay the scan beams with a desired magnification and minimal loss ofpower. However, splitting the power of a single laser into multiple scanbeams greatly reduces the power that can be delivered per unit time to agiven spot on an object, such as a label, thereby affecting thethroughput of a direct scan system. In addition, Tamkin does not addressthe challenges of achieving the long depth of focus required in anautomatic “on-the-fly” labeling system.

A multiple laser diode array may be used in a direct write application,rather than splitting a single laser into multiple beams, as disclosedin Landsman, U.S. Pat. No. 6,640,713. However, unless the laser diodearray can be placed immediately adjacent the light markable medium, asis the case in writing produce labels on the fly, effective delivery ofthe laser light to the medium remains a challenge.

Johnson, U.S. Pat. No. 6,177,980 (hereinafter “Johnson”), discloses anoptical system that couples an array of miniature lens elements, orlenslets, with an image projection system in a low resolution, largefield microlithography application. Johnson modulates the expanded beamof a single diode laser source using a grating light valve or an arrayof micromirrors. The modulated light is then focused by an array oflenslets into widely spaced point images. The beam separation betweenthe lenslets in Johnson is substantially wider than the focused spot,which requires a writing strategy that is not suitable for high-speed,in-line, web-fed processes. While Johnson discloses the use of an afocalsystem with an array of lenslets in a direct writing application, itdoes not address the aforementioned challenges that exist in the designof a direct write imaging system in which the position of the imageplane may change significantly with time, the initial quality of thebeam is poor, as in the output of a multi-mode diode laser, theillumination power of the beam must be high, and a physically compact,cost effective optical package is desirable.

Accordingly, there is a need for an improved optical system forphotosensitive printing by direct writing with a laser beam on a lightmarkable medium, wherein the position of that medium may varysignificantly, the illumination power is high, and the optical systemshould be compact and cost effective.

SUMMARY

An imaging system is disclosed.

In a first respect the imaging system includes an array of lightsources, an array of lenses corresponding to the light sources andhaving optical axes substantially parallel to one another. The lensesproduce collimated output beams. An afocal optical relay having anoptical axis substantially parallel to the optical axes of the lenses isalso included, the array of lenses being positioned relative to theafocal optical relay so as to foiin an optical system that produces animage of each collimated output beam on an image plane, each imagehaving a prescribed depth of focus and spot size.

In a second respect the imaging system includes an array of lasers, thearray of lasers producing an array of respective laser beams. It furtherincludes an array of lenses corresponding to, and disposed at a selectedlocation relative to, the array of lasers so as to produce magnifiedimages of the respective laser beams. An optical relay is disposed at aselected location relative to the array of lenses, so as to produce, atan image plane, images of the respective laser beams, wherein the imagesmeet a selected blur criterion.

A system for writing information on a light-sensitive label is alsodisclosed. The system includes an array of light sources that producesan array of light beams, and an array of lenses corresponding to thelight sources for directing the light beams toward an image plane. Alabeling apparatus is provided for positioning the light-sensitive labelat the image plane. An optical relay disposed between the source arrayand the labeling apparatus produces a magnified image of the light beamson the light-sensitive label so as to expose the label and thereby writea pattern thereon.

Methods of imaging and of writing information on a light-sensitive labelare also disclosed.

It is to be understood that this summary is provided as a means forgenerally determining what follows in the drawings and detaileddescription, and is not intended to limit the scope of the invention.Objects, features and advantages of the invention will be readilyunderstood upon consideration of the following detailed descriptiontaken in conjunction with the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be readily understood from thefollowing detailed description in conjunction with the accompanyingdrawings. To facilitate this description, like reference numeralsdesignate like structural elements. Embodiments of the invention areillustrated by way of example and not by way of limitation in thefigures of the accompanying drawings.

FIG. 1 is a perspective view of an automatic produce labeling apparatusin which a laser beam is used to write coded information on amulti-layer thermally-sensitive label.

FIG. 2 is a side view of a portion of a bellows with a label attachedthereto and aligned with the optical axis of a preferred embodiment ofthe optical system disclosed herein.

FIG. 3 is a schematic side view of an array of laser beams produced by asource array of laser diodes and collimated by an array of microlenses.

FIG. 4 is a schematic end view showing the geometry of custom-fabricatedmicrolens array of FIG. 3.

FIG. 5 is a detailed end view of a lens portion of a single lensletwithin the microlens array shown in FIG. 3

FIG. 6 is a cross-sectional side view of a single lenslet within themicrolens array shown in FIG. 3.

FIG. 7 is a layout diagram for a preferred embodiment of an opticalsystem disclosed herein, showing example marginal rays from a singlelaser diode as they propagate through the system.

FIG. 8 is an unfolded side view of the optical system of FIG. 7, showingonly the three powered mirrors, in which concave surfaces of the firstand third mirrors, and a convex surface of the second mirror, arevisible.

FIG. 9 is a thin lens schematic for the three-mirror afocal portion ofthe optical system of FIG. 7, showing chief and marginal rays emanatingfrom a single, representative, laser diode at the center of the diodearray, and propagating through the entire optical system.

FIG. 10 is a side view of a Gaussian laser beam profile showing thechange in beam width with propagation.

FIG. 11 is a wave optics illustration of the effect of multi-modeoperation of a diode laser on the waist of a Gaussian beam produced bythe laser when collimated by a lenslet.

FIG. 12 is a plot of the width of a laser beam as a function of distancefrom a laser source.

FIG. 13 is a plot of the location of an output waist of a laser beam asa function of the location of its input waist.

FIG. 14 is a pictorial view illustrating misalignment of a laser beamspot with respect to a target label.

FIG. 15 is a reproduction of the optical layout diagram of FIG. 7,further showing placement of a label edge sensor at the input to anafocal optical relay.

FIG. 16 is a schematic of a dichroic beamsplitter within the label edgesensor of FIG. 15.

FIG. 17 is a reproduction of the optical layout diagram of FIG. 7,further showing placement of a detector for monitoring laser powerduring calibration of an afocal optical relay.

DETAILED DESCRIPTION

As mentioned above, an advantage in using a direct-write laser systemfor creating product labels is that the label information may be changed“on-the-fly” according to variations in the product, such as size. Forexample, instead of sorting a batch of fruit by size prior to labeling,individual fruits may be labeled immediately after measuring. In anembodiment of the produce labeling method and apparatus of Hirst,referred to and incorporated herein by reference in its entirety, alabel is acquired by a bellows from a strip of removable labels, exposedto a light beam that causes a pattern of light to be written through thelabel and onto the front surface of the label, and then applied by thebellows to an individual item of produce. (Hirst, FIGS. 1A and 1B;Hirst, col. 3, lines 45-59).

Such a method and apparatus, and the labels used therewith, presentseveral challenges in the design of an optical system for writing on thelabel in the most effective way. One challenge arises because thelongitudinal position of the label may vary significantly as the bellowsrotates into position to apply the individual label onto the produce.Consequently, the consistency of the spot size written on the labeldepends, in part, on the depth of focus of the light beam and, in part,on the quality of the light beam. Another challenge arises because thebeam of light is generally required to be of an intensity sufficient toexpose the photosensitive media adequately. A further challenge is toproduce an intense, high quality beam with a relatively long depth offocus in a physically convenient, cost-effective package.

Turning to FIG. 1, a labeling apparatus 40 is used to measure, and toimmediately apply a label 41 to a product 42 being processed in aproduction line 44. In this example, size or other data about product 42is gathered by a sensor 46, and transmitted to a laser coding device 48that emits a laser beam 50 having an optical axis 52. Labeling apparatus40 transfers an adhesive-backed blank label 41 from a roll 54 of blanklabels 41 onto a bellows tip 56 of a rotary-mounted bellows 58. Uponacquiring label 41, bellows 58 holds the label 41 in place bymaintaining low pressure at the interface between label 41 and bellowstip 56. As bellows 58 rotates along a curved path 59 toward productionline 44, label 41, preferably of a multi-layer thermochromic type,passes through the optical axis 52 of laser coding device 48 and isexposed to laser beam 50. Laser beam 50 is directed to propagate throughan optical conditioning device 60, such as a lens system shownschematically in FIG. 7. Optical conditioning device 60 conditions laserbeam 50 so that it is suitable to accurately write coded labelinformation directly onto label 41. As the rotary-mounted bellows 58continues through its rotation, bellows 58 applies label 41 onto product42, and repeats the cycle just described.

In a commercial application of such a produce-labeling system, asignificant challenge is posed by the need for accurate timing,processing speed, and the need to focus an image accurately onto amoving target. For example, the labeling apparatus described in Griffenat paragraphs 114-120, the disclosures of which have been incorporatedby reference above, is able to sustain a product throughput of 720 itemsof produce per minute. It is therefore desirable for the laser beamimage projected onto label 41 to have a large depth of focus so that theimage will remain in focus and retain its magnification throughout asmuch of the bellows' motion as possible, as indicated in FIGS. 6-8 andin paragraphs 63-64 of Griffen, the disclosures of which have beenincorporated by reference above. However, some depth of focus may besacrificed in favor of high power to expose the relatively large area ofthe label 41, which is about 20 mm wide. Characteristics of single laserdiode sources or laser diode arrays suitable for use in such a producelabeling system are given, for example, in Griffen at paragraphs 0119and 0120. They include wavelengths between 800 and 1600 nm and powerlevels of about 500 mW per laser diode.

As shown in FIG. 2, the bellows 58 in an automated labeling system asdescribed above, and in Hirst, FIGS. 1A-1B, and in Griffen, FIGS. 6-8,has a bellows tip 56 that pneumatically attaches to a light-sensitivelabel 41, acquires it from a backing material (not shown), moves itthrough the optical axis 52 of the optical beam conditioner 60, andapplies it to an item of produce, as previously explained. Griffendescribes, in FIGS. 9A-9B, and in paragraphs 65-66, an example of aparticularly advantageous embodiment of a light-sensitive label 41 thatcomprises a three-layer structure shown in FIG. 2, the disclosures ofwhich have been incorporated by reference above. The label 41 preferablyhas a translucent adhesive coating 62, a back, translucent substratelayer 64, a middle, light absorbent layer 66, and a front, thermochromiclayer 68, arranged in that order, such that when a beam of lightilluminates the back of label 41 it passes through adhesive coating 62and substrate layer 64 to absorbent layer 66, in which the radiantenergy is transformed into heat, which then causes thermochromic layer68 to change color wherever it is exposed to light from the back of thelabel. Such a complex, multi-layer label comprising different materialsmay itself be treated as an optical system characterized by a pointspread function, separate from, and in addition to, a point spreadfunction characterizing optical conditioning device 60. As the bellows58 travels through the optical axis 52, the position of the label 41along the optical axis 52 varies over time, primarily due toinconsistency in the radial extension of bellows 58, but also due torotation of the bellows 58 along curved path 59, variation in thesurface shape of label 41, and other factors. This variation in theposition of label 41 is represented by Δz in FIG. 2. Consequently, towrite a sharp image on label 41 consistently, the depth of focus of theoptical system should be at least as long as Δz.

In addition to the disclosures of Hirst and Griffen, incorporated byreference in their entirety, including those particular sections citedabove as, the present disclosure comprises a novel optical system designthat performs the functions of laser coding device 48 and opticalconditioning device 60 and the combination of that optical system withautomatic produce labeling apparatus 40. The optical system comprises alaser diode source array that generates an array of laser beams, amicrolens array that individually collimates the laser beams, and anafocal optical relay that conditions the laser beams and produces laserspots that meet the requirements of a particular application such as theproduce labeling application.

FIG. 3 shows a close-up side view of a laser diode and microlens sourcearray assembly 90. Source array assembly 90 comprises a light sourcearray component 100 and a lens array component 105 spaced apart at aselected distance. Light source array component 100 includes a powersupply (not shown), and, according to a preferred embodiment, an arrayof laser diodes 102 that produce an array of laser beams 104 propagatingalong substantially parallel axes. Laser diodes 102 are preferablyaddressable, programmable light sources, having output powers that maybe individually modulated by varying the current supplied to each diodein the array. Laser light produced by source array 100 preferably has alaser wavelength of about 980 nm, the nominal output power level of eachof about 300 laser diodes 102 is about 500 mW, and laser diodes 102 arespaced apart by about 125 microns. Laser diode arrays of the typedescribed herein can be obtained, for example, from OSRAM OptoSemiconductors, Inc. of Sunnyvale, Calif. and Laser 2000 GmbH in Munich,Germany. Lens array component 105 preferably comprises a collimatingmicrolens array 106, the elements of which are individual lenslets 107having substantially parallel optical axes, the lenslets 107 thusproducing a collimated array of laser beams 104.

FIG. 4 shows an end view of a preferred embodiment of a customizedmicrolens array 106, having an array length 202 of about 35 mm and anarray width 204 of about 5 mm. Microlens array 106 is acustom-fabricated device manufactured by companies such as RochesterPhotonics Corporation of Corning, N.Y. Microlens array 106 is fabricatedby construction of a repeating linear pattern of microlens arrayelements, or lenslets 107. Lenslets 107 are disposed adjacent to oneanother, with a center-to-center spacing distance 208 of about 125microns, forming a one-dimensional, vertical row 210, of about 280lenses. As shown in FIG. 5, which is an enlarged end view of a singlemicrolenslet 107, centered within each lenslet 107 is an individualtransparent microlens 212, having a lens diameter 214 of about 500microns. Microlenses 212 may be replicated in polymer, solgel, or etchedinto the glass substrate. In a preferred embodiment, a pair of clearaperture (i.e., transparent), aspherical, convex, conic section polymermicrolenses 212 are used to control aberrations, instead of using aunitary cylindrical lens design followed by a single-surface array, asis common in existing laser array systems.

FIG. 6 shows a single lenslet 107 in cross section. Lenslet 107 isfabricated on a fused silica (glass) substrate 216 about 1 mm thick,with an index of refraction of about 1.45. Flanking substrate 216 aretwo parallel photopolymer base layers 218, about 50 mm thick, with anindex of refraction of about 1.54. Each polymer microlens 212 preferablyhas an aspheric, hyperboloid shape, and is formed so as to protrudelaterally by a height 219 of about 40 microns from either a frontsurface 220, or a rear surface 222 of polymer base layer 218. At thecenter of each lenslet 107 in the vertical row 210, one bi-hyperboloidpolymer lens 212 protrudes from front surface 220, and another lens 212protrudes from rear surface 222. Thus, an individual laser beam 224propagating from left to right in FIG. 6 at perpendicular incidence tothe plane of the microlens array 106 passes through a pair ofhyperboloid lenses 212, as well as through polymer base layers 218 andglass substrate 216 sandwiched between the lens pair.

In FIG. 7, an all-reflective afocal optical relay system 300 is shownpositioned between an object plane 301, at which the source arrayassembly 90 is positioned, and an image plane 302, which is co-locatedwith a target label 303. Alternatively, while an all-reflective systemis preferred to minimize power losses in transmission, it is to beunderstood that optical relay system 300 may be refractive, comprisinglenses instead of mirrors, without departing from the broadestprinciples of the invention.

An output image 304, of the array of laser beams 104, is formed at theimage plane 302, the image 304 comprising individual laser beam spots,each having a spot size 308. From a geometrical optics point of view,rays of light comprising each laser beam 224 produced by a given laserdiode 102 of the source array 100, are collimated by a given lensletarray 107, and then the collimated beams propagate through a series ofpolished mirrors 310-320, some of which are powered, to produce afractionally magnified output image 304 of the laser beam spot at imageplane 302. Because the chief rays enter and leave the afocal opticalrelay system 300 parallel to the optical axis, the magnification doesnot change with defocus. The depth of focus is strictly determined bythe wave optics characteristics of the focused laser spot at the finalimage plane. This is one advantage of the preferred system design shown.

As the rays comprising laser beam 224 propagate through optical relaysystem 300, they are deflected by each of mirrors 310-320 along a foldedoptical path, according to the law of reflection, which dictates thatthe angle of reflection equals the angle of incidence with respect to anormal to the surface of the mirror at the point of reflection. Thefirst two mirrors shown, 310 and 312, are preferably flat mirrors,neither concave nor convex. Therefore they do not alter the profile ofbeam 224; rather, they direct the beam into the tilted mirror system.Mirrors 314, 316, and 318 are preferably spherical powered mirrorscomprising a three-mirror afocal system 319. A three-element afocalsystem is used instead of a two-element system to further controlaberrations. An output mirror 320 is preferably a flat mirror, angled soas to direct conditioned laser beam 224 toward target label 303 at theimage plane 302. Mirrors 314-318 may be aspheric when the reductionratio becomes large, causing the NA to exceed 0.05. The three-mirrorsystem 319 serves to minimize aberrations so that the system performanceremains diffraction-limited, rather than aberration-limited.

Referring to FIG. 8, which shows an unfolded, front view of thethree-mirror afocal system 319, in a preferred embodiment, a firstmirror 314 and a third mirror 318 are preferably positive poweredmirrors which increase the size of output image 304; mirror 316 ispreferably a convex, negative powered mirror which decreases the size ofoutput image 304. Mirrors 314-318 thus cooperate to condition the laserbeam 224 to produce the desired output image 304, having a desired spotsize 308. A principal characteristic of the optical system design thatincludes microlens array 106 is to remove the punctile nature of laserdiode emission, relative to the array element spacing, thereby causingeach of the laser beams 224 to diverge enough that overlapping spots areformed on the image plane 302. Microlens array 106 effectively reducesthe numerical aperture (NA) of the output of each laser diode 102 by atleast about a factor of 10, thereby relaxing constraints on the designof afocal optical relay system 300.

It is important to note that an afocal system maintains themagnification of the output image 304 even if the object plane 301 orthe image plane 302 is shifted. This is important because, as theposition of the bellows tip 56 shifts through the depth of focus, due torotation, vibration, and other mechanical errors, the lateral positionof the image will not change or become distorted during the direct writeoperation.

The final magnification of the output image 304 may be tuned by varyingthe relative positions of the mirrors within afocal optical relay system300. A prescription for a suitable afocal optical relay system 300 isdetailed in Table 1, and illustrated by FIG. 9.

TABLE 1 Prescription for afocal optical relay system 300. Radius ofCurvature, Optical Element Position, mm mm Object infinity N/A 1^(st)powered mirror −190.97  −80.78 (concave) 2^(nd) powered mirror  −86.54 21.19 (convex) 3^(rd) powered mirror −159.42 −156.67 (concave) Imageinfinity N/A

In FIG. 9, an unfolded, reduced linear ray trace diagram of a laser beam224 illustrates optical properties of the preferred embodiment ingreater detail. FIG. 9 shows a thin lens representation of a lenslet 107located at object plane 301, the first powered mirror 314 having focallength f1, the second powered mirror 316 having focal length f2, and thethird powered mirror 318 having focal length f3, distributed in thatorder to output image 304, along the optical axis 52 of the opticalconditioning device 60. Mirrors 314-318 form an afocal system; however,the object is not actually located at infinity.

FIG. 9 shows a chief ray 321 representing laser beam 224 that entersmirror 314 from the left, parallel to the optical axis 52, and exitsfrom mirror 318 to the right, again parallel to the optical axis 52.Each such chief ray generated by each of laser diodes 102 then follows apath through the optical relay afocal system 300 like that of therepresentative chief ray 321 to the final image 304. A marginal ray 322represents half of the extent of the width of laser beam 224. In apreferred embodiment, fractional magnification occurs, so the laser beamwidth at the entrance to the afocal system is greater than the laserbeam spot size 308 at the output of the afocal system.

An important feature of the preferred embodiments disclosed herein isthe positioning of the laser diode source array 100 with respect to themicrolens array 106, so as to provide both the desired the depth offocus and beam width at image plane 302, while also providing themaximum optical power. Lenslets 107 limit the amount of light collectedfrom each laser diode 102, thus limiting the size of the laser beam 224that exits each lenslet 107. At the same time, for use in on-the-flylabel writing as described herein, and for other high speed imagingapplications, it is important that the images of the laser beam spotscorresponding to adjacent laser diodes 102 overlap at the image plane302. This is to be able to produce continuously written areas on label41, whereby any spaces between the written areas are the result ofturning off one or more laser diodes 102. Without a microlens 212, thespot sizes on the facets of the laser diode source 102 are re-imagedonto the image plane 302. These spots, about several microns indiameter, are thus very small compared to the center-to-center lensletspacing distance 208. Use of a micro lens 212 “collimates” the beam fromeach laser, yielding larger spots, about the same size as the 125 microncenter-to-center spacing distance 208. Since, as a practical matter,light from one laser diode source 102 should be captured by only onelenslet 107, the actual image spot size 308 is slightly smaller than thespacing distance 208, and the laser beams 224 exiting two adjacentlenslets 107 will not immediately overlap. However, adjacent laser beams224 can be caused to overlap at some distance away from lenslet 107,because the laser beams 224 spread out as a function of distance (d)according to equation (1). Therefore, the image to be placed on imageplane 302 is not that of the plurality of laser beams 224 directlyexiting lenslets 107; rather, it is an image located at some distanceaway from the microlens array, at which adjacent laser beams 224 overlapsufficiently.

Turning to wave optics, FIG. 10 shows a more realistic representation ofthe shape of a beam of light that propagates through the optical system.It will be recognized by a person having skill in the art that theoutput of laser diode 102 ordinarily is a Gaussian laser beam 330, andthat a lens or powered mirror of focal length f configured to collimateor focus Gaussian laser beam 330 produces a waist, or minimum width,ω_(om), at waist plane 332 in the image space of that lens or mirror.The laser beam width ω_(m) expands along the optical propagation axis 52of laser beam 330 as a hyperbolic function of distance z from the waistω_(om), such that the width is a function of the waist ω_(om), distancez, focal length f, wavelength λ, and a mode parameter M given by:

$\begin{matrix}{{\omega_{m}\left( {\lambda,\omega_{om},z,M} \right)}\text{:}{= \omega_{om} \cdot \left\lbrack {1 + \left( \frac{\lambda \cdot z \cdot M^{2}}{\pi \cdot \omega_{om}^{2}} \right)^{2}} \right\rbrack^{.5}}} & (1)\end{matrix}$

The depth of focus is, then, a distance b in front of and in back of thewaist ω_(om) within which an acceptable blur criterion is satisfied, asshown in FIG. 10. For a given laser beam waist ω_(om), the depth offocus b of the optical system is determined by the width of the laserbeam ω_(m) and the mode content of the laser source, as described by itsM² value. The depth of focus is thus independent of the position of theafocal relay system 300 relative to the diode lenslet source arrayassembly 90. In a preferred embodiment of the optical system disclosedherein, as used as used in the produce labeling application, a distance2 b, equal to twice the depth of focus, should exceed the variation inthe label position, Δz, shown in FIG. 2.

FIG. 11 shows the effect of multi-mode operation of laser diode 102 onthe laser beam width and consequently on the depth of focus, b, asGaussian laser beam 330 passes through lenslet 107. With the use of beamoptics, a laser beam 338 representing the central mode of Gaussian laserbeam 330, and a laser beam 340 representing an edge mode of Gaussianlaser beam 330, it can be seen that, as compared to single modeoperation, in multi-mode operation (M²>1), the expansion of the combinedlaser beam 366 occurs much more rapidly with distance z from the waistthan does the expansion of a single laser beam 368, represented byarrows 342. This rapid expansion is accompanied by a correspondingshrinkage of the depth of focus b at the image plane 302. Preferably,the focal length of the lens is chosen to collect the most light fromeach diode 102. Since the center-to-center spacing distance 208 betweenarray elements is fixed, a lens having a short focal length will collectmore of the diverging light. However, a lens having a short focal lengthwill also yield a narrow collimated output beam. Larger focal lengthsproduce larger spots, but if the focal length becomes too large, lightspills over into adjacent array elements, causing too much overlap.Typically, the desired focal length would be that which matches the NAof the lenslet 107 to the divergence angle of the laser beam 224.However, these two competing goals are balanced to obtain the optimumfocal length.

According to Equation 2, the largest spot waist for an optimum focallength occurs when the laser source 100 is located at the front focus ofmicrolens 312. If the focal length of microlens 312 is chosen so thatthe NA of the lenslet 107 matches the divergence angle of laser diodes102, then the laser beam width ω_(m) (which, at image plane 302 iseffectively the image spot size 208) as a function of laser sourceposition z is shown in the plot in FIG. 12. The laser diode source 102need not be located at the front focus of the microlens 212, butaccording to Equation 2, the largest spot size 308 corresponding to thesmallest focal length occurs when the laser diode source 102 is locatedat the front focus.

$\begin{matrix}{{\omega_{{om}\; 2}\left( {z,f,\lambda,\omega_{om},M} \right)}\text{:}{= \omega_{om} \cdot \frac{1}{\sqrt{\left( {1 - \frac{z}{f}} \right)^{2} + \left\lbrack \frac{\pi \cdot \left( \frac{\omega_{om}}{M} \right)^{2}}{\lambda \cdot f} \right\rbrack^{2}}}}} & (2)\end{matrix}$

Each laser diode 102 could be placed so that the semiconductor facetthat emits the laser light is positioned at the front focal point of itscorresponding lenslet 107, and so that the waist ω_(om) of the laserbeam 224 is at the back focal point of lenslet 107. However, thislocation is also the most sensitive to defocus errors. The output waistlocation d₂ as a function of the input waist location d₁ may be computedaccording to Equation 3, as is shown in FIG. 13:

$\begin{matrix}{{d_{2}\left( {d_{1},f,\lambda,\omega_{om},M} \right)}\text{:} = \frac{f}{1 - \frac{{d_{1} \cdot f} - f^{2}}{d_{1}^{2} - {d_{1} \cdot f} + {\pi \cdot \frac{\left( \frac{\omega_{om}}{m} \right)^{2}}{\lambda}}}}} & (3)\end{matrix}$

In a preferred embodiment, the focal length of the microlens array 106is slightly larger than the optimum focal length used in Equation 2 toobtain the data of FIG. 12, and the focal plane location of the laserbeam is not located at the front focus, which pushes the waist locationat the output to be in the range of about 5 mm-15 mm from the front ofmicrolens array 106. The laser beam then expands from there so thatadjacent beams 224 overlap at some distance away from the lenslet. Theimage of the laser beam spots is then transferred to the image plane 302by the afocal optical relay system 300.

Output image 304 of laser diodes 102 has a predetermined magnificationthat is selected to satisfy the pixel pitch requirement of thedirect-write application. This is illustrated by way of an example, inwhich thermochromic target label 303 is positioned for marking at imageplane 302, and a bar code marking width of 18 mm is needed, with adesired image pixel spacing of about 70 microns. Given that laser beam224 diverges by about 5-10 degrees at full width, half maximum(hereinafter “FWHM”) as it propagates through microlens array 106, itsGaussian beam radius at the output of the microlens array 106 is about62 microns. This translates to a FWHM laser beam spot size 308 at theoutput of the microlens array 106 of about 73 microns. The overallmagnification of the afocal optical relay system 300 is given by theratio of the image pixel spacing (70 microns) to the laser diode arraypitch, in this example, (about 125 microns), yielding a factor of 0.562.Applying this factor to the FWHM laser beam spot size yields a finaloutput laser beam spot size 308 of 41 microns.

Referring to FIGS. 14-16, a label edge sensor 350, shown in FIGS. 15 and16 may be provided for detecting proper centering of the laser beamoutput image 304, as shown in FIG. 14, in which output image 304 hasfinal output laser beam spot size 308, with respect to target label 303positioned on bellows tip 56. In a preferred embodiment, label edgesensor 350 may be inserted between microlens array 106 and the input tooptical relay system 300. Turning to FIG. 16, label edge sensor 350 ispreferably constructed using a red laser beam 352 that is reflected bymulti-layer thermochromic target label 303. Red laser beam 352 is splitusing a 50% dichroic beamsplitter 354, so that half of the red lightforms a reference signal 355 that is deflected by 90 degrees anddirected toward a split detector 356. The other half of the red lightforms a sensing signal 358 that is reflected by a flat mirror 360 so asto propagate alongside laser beam 224 throughout optical relay system300. When sensing signal 358 encounters target label 303, it reflectsand forms a return signal 362. Return signal 362 propagatesanti-parallel to laser beam 224, back along the folded path of opticalrelay system 300 until it again meets flat mirror 360 and dichroic beamsplitter 354, which cooperate to direct the return signal 362 into splitdetector 356. If sensing signal 358 and laser beam 224 are misalignedwith respect to target label 303, at least a portion of sensing signal358 will fail to encounter target label 303, thereby diminishing theintensity of return signal 362. When the intensity of return signal 362is then compared with that of reference signal 355, a mismatch indicatesmisalignment of laser beam 224 on the target label 303.

Referring to FIG. 17, a single power detector 364 for monitoring thepower level of laser beam 224 may be added to afocal optical relaysystem 300. In a preferred embodiment, power detector 364 is placedbehind second mirror 316, which may be specially designed to havepartial transmission, thereby allowing a portion of the light from laserbeam 224, ranging from 0.1% to 0.5%, to be sacrificed and directed intopower detector 364.

Although certain embodiments have been illustrated and described herein,it will be appreciated by those of ordinary skill in the art that a widevariety of alternate and/or equivalent embodiments or implementationscalculated to achieve the same purposes may be substituted for theembodiments illustrated and described without departing from the scopeof the present invention. Those with skill in the art will readilyappreciate that embodiments in accordance with the present invention maybe implemented in a very wide variety of ways. This application isintended to cover any adaptations or variations of the embodimentsdiscussed herein. The terms and expressions which have been employed inthe foregoing specification are used therein as terms of description andnot of limitation, and there is no intention, in the use of such termsand expressions, to exclude equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims that follow.

1. An imaging system, comprising: a source array of light sources; anarray of lenses corresponding to the light sources having optical axessubstantially parallel to one another, the lenses producing collimatedoutput beams; and an afocal optical relay having an optical axissubstantially parallel to the optical axes of the lenses; wherein thearray of lenses is positioned relative to the afocal optical relay so asto form an optical system that produces an image of each collimatedoutput beam on an image plane, each image having a prescribed depth offocus and minimum spot size.
 2. The imaging system of claim 1, whereineach of the light sources has a separately variable output power, andthe light sources are modulated so as to selectively vary theirrespective output powers.
 3. The imaging system of claim 2, wherein thelight sources are programmable laser diodes that may be individuallymodulated by varying a current supplied to the diodes.
 4. The imagingsystem of claim 3, wherein the light sources are arranged in a lineararray.
 5. The imaging system of claim 4, wherein the afocal opticalrelay comprises a series of powered mirrors.
 6. The imaging system ofclaim 5, wherein the powered mirrors comprise a first, concave mirror, asecond, convex mirror, and a third, concave mirror.
 7. The imagingsystem of claim 6, wherein the light sources have a substantiallypunctile structure in comparison to the center-to-center spacing betweenthe lenses, the collimated beams produced therefrom being magnified bythe optical relay such that the images overlap by a selected amount. 8.The imaging system of claim 1, wherein the light sources are laserdiodes and the afocal optical relay comprises a series of poweredmirrors.
 9. The imaging system of claim 1, wherein the light sourceshave a substantially punctile structure in comparison to thecenter-to-center spacing between the lenses, the collimated beamsproduced therefrom being magnified by the afocal optical relay such thatthe images overlap by a selected amount.
 10. An imaging system,comprising: a source array of light sources; an array of lensescorresponding to the light sources, the lenses having optical axessubstantially parallel to one another and being positioned relative totheir respective light sources so as to produce first images of thelight sources; and an optical relay comprising at least one poweredreflective surface, having an optical axis substantially parallel to theoptical axes of the lenses, and being positioned relative to the firstimages of the light sources to produce at an image plane magnifiedsecond images of the light sources, whereby the powered reflectivesurface serves to minimize power loss in the optical relay.
 11. Theimaging system of claim 10, wherein each of the light sources has aseparately variable output power, and the light sources are modulated soas to selectively vary their separate output powers.
 12. The imagingsystem of claim 11, wherein the light sources are programmable laserdiodes that may be individually modulated by varying a current suppliedto the diodes.
 13. The imaging system of claim 12, wherein the lightsources are arranged in a linear array.
 14. The imaging system of claim10, wherein the afocal optical relay comprises a series of poweredmirrors that form an afocal system.
 15. The imaging system of claim 10,wherein the light sources are lasers.
 16. The imaging system of claim15, wherein the powered mirrors comprise a first, concave mirror, asecond, convex mirror, and a third, concave mirror.
 17. The imagingsystem of claim 10, wherein the light sources have a substantiallypunctile structure in comparison to the center-to-center spacing betweenthe lenses, the first images produced thereby being magnified by theoptical relay such that the second images overlap by a selected amount.18. The imaging system of claim 1, wherein the light sources are lasers.19. An imaging system, comprising: a source array of lasers, the sourcearray of lasers producing an array of respective laser beams; an arrayof lenses corresponding to, and disposed at a selected location relativeto, the source array of lasers so as to produce magnified images of therespective laser beams; and an afocal optical relay, disposed at aselected location relative to the array of lenses, so as to produce, atan image plane, images of the respective laser beams, wherein the imagesmeet a selected blur criterion.
 20. The imaging system of claim 19,wherein each lens within the array of lenses has a front focal plane anda back focal plane, and wherein the lasers are disposed at a selectedobject plane relative to the front focal planes of the respectivelenses.
 21. The imaging system of claim 20, wherein each laser beam hasa waist and wherein the optical relay has an object plane located at adistance with respect to the waist such that adjacent images formed bythe optical relay overlap by a selected amount.
 22. The imaging systemof claim 19, wherein each laser beam has a waist and wherein the opticalrelay has an object plane located at a distance with respect to thewaist such that adjacent images formed by the optical relay overlap by aselected amount.
 23. The imaging system of claim 22, wherein the opticalrelay is an afocal system wherein aberrations in the image are minimizedfor an objected located at the object plane.
 24. The imaging system ofclaim 23, wherein the lasers are multi-mode lasers.
 25. The imagingsystem of claim 22, wherein each of the lasers has a separately variableoutput power, and the lasers are modulated so as to selectively varytheir separate output powers.
 26. The imaging system of claim 25,wherein the lasers are programmable laser diodes that may beindividually modulated by varying a current supplied to the diodes. 27.A method of imaging, comprising: providing a plurality of light sources;collimating light from the light sources so as to produce acorresponding plurality of collimated light beams; and afocallyproducing images of the plurality of light beams at an image plane, eachimage having a prescribed depth of focus and minimum spot size.
 28. Themethod of claim 27, wherein the light sources have a substantiallypunctile structure, and the collimated light beams therefrom are causedto overlap by a selected amount at the image plane.
 29. The method ofclaim 28, further comprising magnifying the image a fractional amount.30. The method of claim 27, further comprising modulating the lightsources so as to selectively vary their individual power outputs. 31.The method of claim 28, further comprising arranging the light source ina linear array so that a two-dimensional pattern can be produced bymodulating the light sources while a target is moved through the lightbeams in a direction perpendicular to the axis of the linear array. 32.A method of imaging, comprising: providing a plurality of light sources;collimating light from the light sources so as to produce acorresponding plurality of collimated light beams; and reflectivelyproducing images of the plurality of light beams at an image plane. 33.The method of claim 32, wherein reflectively producing images comprisescausing the plurality of light beams to be reflected sequentially from aplurality of light powered surfaces.
 34. The method of claim 33, furthercomprising magnifying the image.
 35. The method of claim 32, furthercomprising modulating the light sources so as to selectively vary theirindividual power outputs.
 36. The method of claim 33, further comprisingarranging the light sources in a linear array so that a two-dimensionalpattern can be produced by modulating the light sources while a targetis moved through the light beams in a direction perpendicular to theaxis of the linear array.
 37. The method of claim 32, wherein the lightsources are laser light sources.
 38. A method of imaging, comprising:providing a plurality of laser light sources having respective outputs;separately producing a plurality of respective images of the outputs ata selected location; and afocally producing a single image of theoutputs at an image plane, wherein the plurality of outputs at the imageplane meet a selected blur criteria.